1. Field of the Invention
The present invention relates to a light deflecting apparatus which incorporates a light deflecting unit such as a rotary polygon mirror. More particularly, the present invention is concerned with a construction for mounting, on a mounting base, an optical box which contains a light source unit, the light deflecting unit and an optical system for converging the light deflected by the light deflecting unit.
2. Description of the Related Art
In general, a light deflecting apparatus used in an image recording apparatus such as a laser beam printer is composed of two parts: namely, an optical box in which optical components of a scanning system including a light source, a rotary polygon mirror and an f.theta. lens are integrated as a unit; and a main part (mounting base) to which the optical box is mounted. In this type of light deflecting apparatus, it is often experienced that the scanning line is deviated from the aimed position due to, for example, inferior precision in machining and installation. Therefore, known light deflecting apparatus of the kind described are provided with an adjusting mechanism for enabling adjustment of the position of the scanning line. Such an adjusting mechanism, for example, is designed to enable adjustment of mounting position of the optical box to the mounting base. The known adjusting mechanisms, however, are not designed to enable the mounting position to be rotated in a deflection plane which is the plane drawn by the light beam deflected by the deflection unit. Thus, the known light deflecting apparatus do not have any means which enable correction of any inclination of the best, or optimum, image surface where good light spots are obtained with respect to the scanned surface such as the surface of a photo-sensitive drum. This is because the above-mentioned inclination can be maintained within a tolerance through enhancing machining and precisely setting the optical components, since about .+-.6 mm offset from the focal position is acceptable to maintain a laser beam spot diameter of 100 .mu.m or so.
In case of modern precision light deflecting apparatus in which a very small spot diameter of 40 .mu.m or so is required, the tolerance of the offset from the focal position is as small as .+-.0.8 mm. Therefore, correction of the inclination of the scanned surface from the best image surface requires very high machining and mounting precision of the optical parts if the correction has to be effected by improvement in the machining and mounting precision alone. To meet such a requirement, machining and mounting has to be conducted with very high precision, with the result that the cost of the apparatus is undesirably raised. Even if the inclination is set small enough to maintain the deviation within the required tolerance of the offset from the focal position, there still exists a problem that the spot diameter is increased beyond the allowable limit at local portions of the scanned surface, because a sufficiently large margin for offset from the focal position cannot be obtained over the entire area of the scanned surface.